ИСТИНА

Mehler reaction results in superoxide radical and hydrogen peroxide appearance in photosynthetic organisms because of an electron transfer to molecule of O2 by carriers of photosynthetic electron transport chain. Oxygen is a weak acceptor of electrons in the chain; that is why oxygenic photosynthesis occurs. However, if electron transport to NADP+ is limited, e.g. in stress conditions such as high light, the electron flow to oxygen becomes significant. It is still unknown what electron carriers reduce O2. Photosystem 1 (PS1) is believed to be a major site of Mehler reaction. Superoxide radical was recently shown to be produced outside and inside thylakoid membrane simultaneously. It allows supposing an operation of two ways of O2 reduction. O2 reduction by FeS clusters of PS1, terminal electron carriers, results in superoxide production outside the membrane. O2 reduction by low-potential carriers of PS1 situated in the body of protein complex has to be the process that leads to superoxide production within the membrane. The last can be carried out by phylloquinone (PhQ) at A1 site. In this work, the hypothesis about PhQ involvement in O2 reduction was checked. PS1 complexes were isolated from wild type strain of cyanobacteria Synechocystis sp. PCC 6803 (PS1WT), and from the mutant lacking menB gene (PS1menB). The mutant menB cannot synthesize PhQ. Plastoquinone (PQ) occupies A1 site in the mutant. PQ has more positive redox potential than PhQ resulting in a decrease of probability of O2 reduction within the hydrophobic core of PS1 in the mutant. Oxygen reduction by PS1 complexes under steady-state illumination was measured using an O2 electrode as an uptake of O2. N,N,N',N'-tetramethyl-1,4-p-phenylene diamine (TMPD) and sodium ascorbate were added as artificial electron donors to P700+. The dependences of O2 uptake rate on light intensity for both complexes were measured in the absence and presence of methyl viologen (Mv). In the absence of Mv, oxygen was the sole electron acceptor from PS1. Under low and moderate light, O2 reduction rate in PS1WT and PS1menB was equal. In both types of complexes the rate increased with increasing in light intensity. Under high light, the rate in PS1menB tended to saturation while the rate in PS1WT continued increasing. In the presence of Mv, a very efficient acceptor of electrons from terminal electron carriers of PS1, electron transport in PS1 resulted in oxygen uptake also, since reduced Mv is quickly oxidized by oxygen. The addition of Mv caused the stimulation of O2 uptake rate in all cases. It implies the existence of a limitation of the electron transfer from terminal carriers of PS1 to oxygen in the absence of Mv. Under high light, the addition of Mv stimulated O2 uptake rate in PS1WT less than in PS1menB. It means that in the absence of Mv there was an additional electron flow to O2 in PS1WT which was absent in PS1menB. It argues that phylloquinone is involved in oxygen reduction in PS1 under high light. The reported study was supported by RFBR, research project 12-04-31219